VIBRATION MEASUREMENT BY PULSE DIFFERENTIAL HOLOGRAPHIC INTERFEROMETRY

A pulsed differential holography technique has been devised to measure structural deformation of materials that are subjected to a wide range of temperatues and other environmental conditions. Continuous wave, time-average laser holography is a method for mapping the displacement of a vibrating structure at ambient temperatures. At elevated temperatures, however, this method is severely limited by distortions induced by convection currents in the atmosphere adjacent to the test structure. The effects of convection currents are eliminated by operating a pulsed laser in a double pulse mode which exposes the hologram two times in quick succession. The timing of the two exposures, in pulsed differential holography, is carefully controlled to be synchronized with the deflection of the object being measured. The optimum time interval between exposures is dependent on several factors such as the number of fringes desired, vibration amplitude and frequency, and temperature of the structure. Too long a period between pulses results in fringes that are perturbated by the convection currents surrounding the object, while the shorter time intervals permit operation at higher temperatures and higher vibration amplitudes or frequencies. The magnitude of the structural deformation is determined by counting the number of interference fringes that appear on the hologram. The first dark fringe corresponds to a deflection of one-quarter wavelength of the laser light being used, the second corresponds to three-quarters of a wavelength, and so on. The larger the slope in the surface of the object, caused by bending or deflection from an impact or other vibration-producing mechanism, the closer the spacing between fringes. Using this technique, vibration amplitudes have been successfully measured on stainless steel panels heated to temperatures from 394 K (250 F) to 13494 K (2050 F).

  • Supplemental Notes:
    • This article appeared in the publication: Analytical Techniques: A Compilation.
  • Corporate Authors:

    National Aeronautics and Space Administration

    400 Maryland Avenue, SW
    Washington, DC  USA  20546
  • Authors:
    • Evensen, D A
    • Aprahamian, R
  • Publication Date: 1975-1

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Filing Info

  • Accession Number: 00128505
  • Record Type: Publication
  • Report/Paper Numbers: NASA CR-2028
  • Contract Numbers: MFS-24402, NPO-11612, MFS-16968
  • Files: TRIS
  • Created Date: Apr 21 1976 12:00AM